To be able to determine the gas law constant, R, the values of P,V,n, and T must be available. The experiment is based on the reaction between magnesium metal and hydrochloric acid to produce hydrogen gas, Equation 1. The volume, pressure, and temperature under which the hydrogen gas is collected will be measured. From the known quantity of magnesium used and the stoichiometry of the reaction the number of moles of hydrogen produced can be calculated.

Mg(s) + 2 HCl(aq) MgCl2(aq) + H2(g) Eq. 1

Since the hydrogen is collected in a eudiometer tube over an aqueous solution the gas pressure in the tube after the reaction has ceased are the sum of the hydrogen gas pressure and the vapor pressure of the water. In order to obtain the pressure of the hydrogen gas, the vapor pressure of the water, PH2O, at the temperature of the measurement must be subtracted from the atmospheric pressure, Patm, Equation 2.

PH2 = Patm – PH2O Eq. 2

In case the liquid levels (step 3) cannot be equalized after the reaction has ceased, a further correction will be required since the pressure of the gases in the tube (hydrogen and water vapor) will not then be equal to the atmospheric pressure. If this is the case, the difference in levels must be measured with a meter stick as accurately as possible. This difference, which represents the desired pressure difference, must be converted to mmHg. This can be accomplished by dividing the measured level difference in millimeters by 13.6 ( the ratio of the densities of Mercury and the aqueous solution). This difference must then be subtracted from the atmospheric pressure. Thus, if the levels cannot be equalized the pressure of hydrogen must be obtained from the following expression(Cornely and Moss, 2001)

PH2 = Patm – PH2O –Plevel difference Eq. 3

Vapor Pressure (P) of Water at Various Temperatures

If the Lab Temperature is not in the chart then the pressure of water vapor must be calculated using the following equation.

ln (P2/P1) = (?Hvap / R) (1/T1 –1/T2) where R = 8.3145 J/mole K

?Hvap = 44.0 kJ/mole